1. Field of the Invention
The present invention relates to drug-eluting medical devices; more particularly, this invention relates to processes for crimping a polymeric scaffold to a delivery balloon.
2. Background of the Invention
The art recognizes a variety of factors that affect a polymeric scaffold's ability to retain its structural integrity when subjected to external loadings, such as crimping and balloon expansion forces. These interactions are complex and the mechanisms of action not fully understood. According to the art, characteristics differentiating a polymeric, bio-absorbable scaffolding of the type expanded to a deployed state by plastic deformation from a similarly functioning metal stent are many and significant. Indeed, several of the accepted analytic or empirical methods/models used to predict the behavior of metallic stents tend to be unreliable, if not inappropriate, as methods/models for reliably and consistently predicting the highly non-linear behavior of a polymeric load-bearing portion of a balloon-expandable scaffold (hereinafter “scaffold”). The models are not generally capable of providing an acceptable degree of certainty required for purposes of implanting the scaffold within a body, or predicting/anticipating the empirical data.
Moreover, it is recognized that the state of the art in medical device-related balloon fabrication, e.g., non-compliant balloons for scaffold deployment and/or angioplasty, provide only limited information about how a polymeric material might behave when used to support a lumen within a living being via plastic deformation of a network of rings interconnected by struts. In short, methods devised to improve mechanical features of an inflated, thin-walled balloon structure, most analogous to mechanical properties of a pre-loaded membrane when the balloon is inflated and supporting a lumen, simply provides little, if any insight into the behavior of a deployed scaffold. One difference, for example, is the propensity for fracture or cracks to develop in a scaffold. The art recognizes the mechanical problem as too different to provide helpful insights, therefore, despite a shared similarity in class of material. At best, the balloon fabrication art provides only general guidance for one seeking to improve characteristics of a scaffold.
Polymer material considered for use as a scaffold, e.g. PLLA or PLGA, may be described, through comparison with a metallic material used to form a scaffold, in some of the following ways. A suitable polymer has a low strength to weight ratio, which means more material is needed to provide an equivalent mechanical property to that of a metal. Therefore, struts must be made thicker and wider to have the strength needed. The scaffolding also tends to be brittle or have limited fracture toughness. The anisotropic and rate-dependant inelastic properties (i.e., strength/stiffness of the material varies depending upon the rate at which the material is deformed) inherent in the material only compound this complexity in working with a polymer, particularly, bio-absorbable polymer such as PLLA or PLGA.
Processing steps performed on, design changes made to a metal stent that have not typically raised concerns for, or require careful attention to unanticipated changes in the average mechanical properties of the material, therefore, may not also apply to a scaffold due to the non-linear and sometimes unpredictable nature of the mechanical properties of the polymer under a similar loading condition. It is sometimes the case that one needs to undertake extensive validation before it even becomes possible to predict more generally whether a particular condition is due to one factor or another—e.g., was a defect the result of one or more steps of a fabrication process, or one or more steps in a process that takes place after scaffold fabrication, e.g., crimping. As a consequence, a change to a fabrication process, post-fabrication process or even relatively minor changes to a scaffold pattern design must, generally speaking, be investigated more thoroughly than if a metallic material were used instead of the polymer. It follows, therefore, that when choosing among different scaffold designs for improvement thereof, there are far less inferences, theories, or systematic methods of discovery available, as a tool for steering one clear of unproductive paths, and towards more productive paths for improvement, than when making changes in a metal stent.
It is recognized, therefore, that, whereas inferences previously accepted in the art for stent validation or feasibility when an isotropic and ductile metallic material was used, such inferences would be inappropriate for a scaffold. A change in a scaffold pattern may effect, not only the stiffness or lumen coverage of the scaffold in its deployed state supporting a lumen, but also the propensity for fractures to develop when the scaffold is crimped or being deployed. This means that, in comparison to a metallic stent, there is generally no assumption that can be made as to whether a changed scaffold pattern may not produce an adverse outcome, or require a significant change in a processing step (e.g., tube forming, laser cutting, crimping, etc.). Simply put, the highly favorable, inherent properties of a metal (generally invariant stress/strain properties with respect to the rate of deformation or the direction of loading, and the material's ductile nature), which simplify the stent fabrication process, allow for inferences to be more easily drawn between a changed stent pattern and/or a processing step and the ability for the stent to be reliably manufactured with the new pattern and without defects when implanted within a living being.
A change in the pattern of the struts and rings of a scaffold that is plastically deformed, both when crimped to, and when later deployed by a balloon, unfortunately, is not as easy to predict as a metal stent. Indeed, it is recognized that unexpected problems may arise in scaffold fabrication steps as a result of a changed pattern that would not have necessitated any changes if the pattern was instead formed from a metal tube. In contrast to changes in a metallic stent pattern, a change in a scaffold pattern may necessitate other modifications in fabrication steps or post-fabrication processing, such as crimping and sterilization.
Scaffolds used to treat coronary vessels experience, for the most part, a primarily radial loading. However, scaffolds intended for peripheral vessels experience a quite different loading, to such an extent that the traditional measure of a stent's fitness for use, i.e., its radial strength/stiffness, is not an accurate measure of whether the scaffold will have sufficient strength to support the peripheral vessel. This is because a peripheral scaffold is placed in a significantly different environment from a coronary scaffold. The vessel size is larger. And there is much more movement of the vessel, especially when located close to an appendage. As such, a scaffold intended for a peripheral vessel will need to be able to sustain more complex loading, including a combination of axial, bending, torsional and radial loading. See e.g. Bosiers, M. and Schwartz, L., Development of Bioresorbable Scaffolds for the Superficial Femoral Artery, SFA: CONTEMPORARY ENDOVASCULAR MANAGEMENT (‘Interventions in the SFA” section). These and related challenges facing peripherally implanted stents and scaffolds are also discussed in U.S. application Ser. No. 13/015,474.
One challenge, in particular, facing a peripheral scaffold is crimping to a balloon and expansion of the scaffold when the balloon is inflated. Problems arise where, on the one hand, the scaffold cannot be crimped to the desired size without introducing structural failure, i.e., fracture, or excessive cracking, either in the crimped state or when expanded from the crimped state by a balloon. On the other hand, a scaffold can be crimped and deployed, yet deploys with non-uniformity in its deployed state. In these cases the scaffold is susceptible to acute or fatigue failure as the irregularly-deployed rings and/or cells, loaded beyond their design limits as a consequence of the non-uniform deployment, have a reduced acute or fatigue life within the vessel.
A film-headed crimper has been used to crimp polymer scaffolds to balloons. Referring to FIG. 8A, there is shown a perspective view of a crimping assembly 20 that includes three rolls 123, 124, 125 used to position a clean sheet of non-stick material between the crimping blades and a metal stent prior to crimping. For example, upper roll 125 holds the sheet secured to a backing sheet. The sheet is drawn from the backing sheet by a rotating mechanism (not shown) within the crimper head 20. A second sheet is dispensed from the mid roll 124. After crimping, the first and second (used) sheets are collected by the lower roll 123. As an alternative to rollers dispensing a non-stick sheet, each metal stent may be covered in a thin, compliant protective sheath before crimping.
FIG. 8B illustrates the positioning the first sheet 125a and second sheet 124a relative to the wedges 22 and a metal stent 100 within the aperture of the crimping assembly 20. As illustrated each of the two sheets are passed between two blades 22 on opposite sides of the stent 100 and a tension T1 and T2 applied to gather up excess sheet material as the iris of the crimping assembly is reduced in size via the converging blades 22.
The dispensed sheets of non-stick material (or protective sheath) are used to avoid buildup of coating material on the crimper blades for stents coated with a therapeutic agent. The sheets 125a, 124a are replaced by a new sheet after each crimping sequence. By advancing a clean sheet after each crimp, accumulation of contaminating coating material from previously crimped stents is avoided. By using replaceable sheets, stents having different drug coatings can be crimped using the same crimping assembly without risk of contamination or buildup of coating material from prior stent crimping.
In light of the foregoing problems, there is a need to improve the uniformity of deployment for a peripherally-implanted scaffold, while maintaining an appropriate balance among a desired retention force and minimal crossing profile for delivery to a target site. And there is a continuing need to address structural integrity of a peripherally-implanted scaffold after repeated axial, bending and radial loading characteristic of a peripheral vessel.